At present, plastic containers (bottles, etc.) made mainly of polyesters such as polyethylene terephthalate (PET) have been extensively used as containers for teas, fruit juices, carbonated beverages, etc. Among these plastic containers, plastic bottles of a small size have increased in proportion year by year. In general, as the size of a bottle is reduced, a surface area thereof per unit volume of contents therein tends to increase. Therefore, a gustoish period of contents in the small-size bottles tends to be shortened. In recent years, beer susceptible to influences of oxygen and light as well as hot tea which are filled in a plastic bottle have been put on the market. Thus, with the recent tendency that the plastic containers are used in more extensive applications, the plastic containers have been required to be further improved in gas-barrier property against oxygen, carbon dioxide, etc.
To provide plastic bottles having a good gas-barrier property, there have been developed multilayer bottles produced from a thermoplastic polyester resin and a gas-barrier resin, blend bottles, barrier-coated bottles produced by forming a carbon coat, a deposited coat or a barrier resin coat onto a single layer bottle made of a thermoplastic polyester resin, etc.
The multilayer bottles, for example, those bottles produced by subjecting a three- or five-layer preform (parison) obtained by injecting a thermoplastic polyester resin such as PET for forming innermost and outermost layers thereof and a thermoplastic gas-barrier resin such as poly-m-xylyleneadipamide (polyamide MXD6) into a mold cavity, to biaxial stretch blow molding, have been put into practice.
Further, resins having an oxygen-capturing function which are capable of capturing oxygen within a container while preventing penetration of oxygen into the container from outside have been developed and applied to multilayer bottles. The oxygen-capturing bottles are suitably in the form of a multilayer bottle including a gas-barrier layer made of polyamide MXD6 in which a transition metal-based catalyst is blended, from the viewpoints of oxygen-absorbing rate, transparency, strength, moldability, etc.
The above multilayer bottles have been used as containers for beer, tea, carbonated beverages, etc., because of their good gas-barrier property. When the multilayer bottles are used in these applications, contents filled therein can maintain a good quality with an improved shelf life. On the other hand, the multilayer bottles tend to undergo delamination between different resin layers, for example, between the innermost or outermost layer and the intermediate layer, resulting in significant damage to their commercial value.
To solve the above problems, there is disclosed the method of blending polyamide MXD6 with nylon 6 and nylon 6I/6T to prevent crystallization of a barrier layer in a multilayer bottle, reduce a crystallization rate of the polyamide or transform the polyamide into uncrystallizable polyamide, thereby improving a delamination resistance of the bottle (refer to Patent Document 1). However, in this method, in order to prevent crystallization of the polyamide MXD6 and reduce a crystallization rate thereof, a considerable amount of nylon 6 and nylon 6I/6T having a relatively poor gas barrier property as compared to the polyamide MXD6 must be added thereto. Therefore, the resultant multilayer bottle exhibits a poorer gas barrier property than those using the polyamide MXD6 only and, therefore, fails to improve a shelf life of contents therein to a sufficient extent. In addition, when mixing a transition metal-based catalyst in the polyamide for compensating the poor oxygen-barrier property, there arises such a problem that addition of the catalyst causes increase in costs. Also, even when adding the transition metal-based catalyst, the resultant bottle is not improved in carbon dioxide-barrier property and, therefore, still unsuitable as a container for beer, carbonated beverages, etc. In the method described in the Patent Document 1, these polyamides must be melt-blended with each other using an extruder prior to molding of the bottle, resulting in increased production costs.
Patent Document 1: USP 2005/0009976A